1. Field of the Invention
The subject matter disclosed generally relates to the field of semiconductor image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders may contain electronic image sensors that capture light for processing into a still or video image, respectively. There are two primary types of electronic image sensors, charge coupled devices (CCDs) and complimentary metal oxide semiconductor (CMOS) sensors. CCD image sensors have relatively high signal to noise ratios (SNR) that provide quality images. Additionally, CCDs can be fabricated to have pixel arrays that are relatively small while conforming with most camera and video resolution requirements. A pixel is the smallest discrete element of an image. For these reasons, CCDs are used in most commercially available cameras and camcorders.
CMOS sensors are faster and consume less power than CCD devices. Additionally, CMOS fabrication processes are used to make many types of integrated circuits. Consequently, there is a greater abundance of manufacturing capacity for CMOS sensors than CCD sensors.
Both CCD and CMOS image sensors may generate inaccurate image data because of dark current in the pixel and a variation of the dark current from pixel to pixel. Each pixel of an image sensor pixel array provides an output voltage that varies as a function of the light incident on the pixel. Unfortunately, dark currents add to the output voltages. The dark current and its variation from pixel to pixel degrade the picture provided by the image system.
Dark current varies from one pixel to another, thus making it impossible to use a single dark current estimate for the entire pixel array to subtract from all the output voltage values. Dark current also tends to have an exponential dependence on temperature such that its magnitude doubles for every 8 degrees Kelvin. Consequently, a single dark frame captured at a different temperature than the light frame (the actual picture frame) does not accurately represent the dark signals embedded in the light frame and require some sort of scaling. Furthermore, it is common to have temperature variations across the substrate leading to different average dark current at different locations of the pixel array. Additionally, if the temperature of the ambience changes, different locations of the pixels array may have different changes of temperatures, thus leading to different change ratios of dark current at different locations. All these factors make it difficult to use estimate dark signals accurately across the pixel array using a one-time captured dark frame that is scaled and subtracted from the light frames.
There have been efforts to compensate for the dark currents of the pixel array. U.S. Pat. No. 5,926,214 issued to Denyer et al. discloses a dark current compensation scheme where a dark frame is taken contemporaneously with a light frame. Both the dark frame and the light frame are captured at the same temperature. The dark frame is subtracted from the light frame to substantially remove dark signals from the output of the sensor. Unfortunately, such dark frame approaches require the closing of a camera shutter or some other means to prevent light from impinging upon the sensor. Closing the camera shutter contemporaneously with each picture increases the shot-to-shot delay of the camera.
U.S. Pat. No. 6,101,287 issued to Corum, et al. and U.S. Pat. No. 5,278,658 issued to Takase each disclose a scheme where a single dark frame is captured and certain “dark” pixels of the array are used to generate a dark signal reference for scaling the dark frame. The dark pixels are optically shielded from the ambient light. The dark frame is scaled with a ratio of the average dark signal of the dark pixels in the light frame, to the dark frame. Unfortunately, some light still strikes the dark pixels so that the dark pixel outputs are not truly representative of the dark signals. Additionally, the dark signals of interior pixels scale differently than the peripheral dark pixels.
U.S. Pat. No. 6,144,408 issued to MacLean discloses a scheme that does not rely on periphery dark pixels and can generate multiple scaling factors for interiors of the pixel array. In this scheme, certain pixels where dark currents are founded to be strongest are selected and organized into groups. An average Laplacian is calculated for the dark frame for each group. Laplacian basically measures the amount of “spiking” of voltage value at the pixel relative to its neighbors. Subsequently, in the light frame average Laplacian is calculated for each group again. The ratio of average Laplacian in the light frame to that in the dark frame becomes the scaling factors across the entire pixel array from those groups of selected pixels. This technique is relatively inadequate because picture details at the vicinity of those selected pixels interfere with the accuracy of the scaling factor calculation.
There is therefore a need to use a single dark frame to estimate and subtract dark signals for an entire pixel array despite changes in ambient temperature and variations of temperature across the pixel array.